1. Field of the Invention
This invention relates to a speed detecting apparatus, and more particularly to a speed detecting apparatus which can detect the speed of a body which is moving at low speed.
2. Description of the Prior Art
In many fields, a speed detecting apparatus is employed to detect the speed of a moving body which moves linearly, curvedly or rotatively. Hereinafter, description will be made using a rotating body as a typical example of a moving body. It is usual to use a tacho-generator or an encoder in the detection of the speed of a rotating body.
When an encoder is used in a speed detecting apparatus, a signal output from the encoder is subjected to a frequency-speed conversion to detect the speed of a moving body. In other words, the speed of a moving body is detected on the basis of the number of pulses output from the encoder within a predetermined period of time. The applicant has proposed a speed detecting apparatus in which a pulse signal output from an encoder is divided and the time intervals of the divided signals are detected by the use of a reference clock signal so that the detected period of the pulse signal is used to determine the speed of a moving body.
When a rotating body is servo-controlled to rotate at a predetermined speed which is very low, for example, one rotation per several tens of seconds or several minutes, or one rotation per day, the rotational speed of the rotating body must be detected with accuracy at short-time intervals. It is substantially impossible for a tacho-generator to detect such a very low speed with accuracy.
In the frequency-speed conversion of a signal obtained from a body rotating at such a very low speed, the number of output pulses per unit time is extremely small. Even when an encoder with a high resolution is used, therefore, it is almost impossible to detect a speed with accuracy at a predetermined time interval.
In above-mentioned our prior apparatus, a speed is detected at time intervals each corresponding to one period, that is, at the period of the pulse signal output from an encoder. Therefore, when the speed is maintained at a certain level during a short period of time and the signal output from a detecting element has a substantially sinusoidal waveform as shown in FIG. 7, the our prior apparatus can detect the speed within one period H with accuracy. When the speed frequently changes during a short period of time, the signal output from the detecting element has a waveform shown in FIG. 8. Thus, the our prior apparatus can detect the average speed in one period H, but cannot detect the actual speed at each moment, resulting in that the rotating body cannot be servo-controlled to rotate at the predetermined speed.
In a speed detecting apparatus of the invention, detection signals of different phases are scanned to be output as a scanned detection signal having periodicity. When a moving body is stopped, the period of the scanned detection signal is the same as that of the scanning period. When a moving body moves in the phase-leading direction, the period of the scanned detection signal is lengthened by the extent which is offset by the movement in the phase-leading direction. When a moving body moves in the direction reverse to the phase-leading direction, the period of the scanned detection signal is shortened. The speed of the moving body can be detected on the basis of the data including the period of the scanned detection signal, the scanning period, and the distance which the moving body moves in a single period of the phase. Thus, the speed can be detected at each period of the scanned detection signal determined by the scanning period, resulting in that the speed of the moving body can be detected even when it moves at a low speed.
When the scanning period is set short, the detection interval becomes short so that the rapid response of detection is realized and that the detection range becomes large. In this case, however, the accuracy and resolution of the speed detection are relatively poor. On the other hand, when the scanning period is set long, the speed detection is relatively poor in the response and the detection range, but is excellent in resolution.
According to the invention, the scanned detection signal is divided so that the speed can be detected at a desired detection interval. It is selectable whether the detection period is set long to perform the speed detection with high accuracy or it is set short to perform speed detection with excellent response characteristics.